One of Britain's biggest power companies is pushing up prices by an expected 15 per cent or more for gas and 10 per cent for electricity.

The move by npower will add more than £100 a year to average bills for four million customers. It comes as snow sweeps in and one of the coldest winters in 20 years is forecast.

Experts immediately warned it was certain to be followed by all the other major energy providers.

Npower, which is owned by German group RWE, confirmed the decision last night - just hours after denying it to the Mirror.

Although npower was still refusing to give precise details, the predicted increases would add around £80 to the typical npower gas bill of £552 and £38 to the average electricity bill of £377.

The company, which sponsors English Test cricket, had increased online prices by 17 per cent the week before Christmas.

This followed a similar move by British Gas, which raised its market tracker price by 13 per cent for gas and 15 per cent for electricity.

The power companies fear soaring oil prices, which this week smashed past U100 a barrel and yesterday hit a new high.

But industry watchdog Energywatch said the suppliers were using this as an excuse. A spokesman said: "The gas they are selling now will have been bought months ago when the wholesale price was low.

"This is yet more evidence of how quick they are to push prices up at the first excuse.

"Yet it took them several months to pass on the benefits of falling wholesale prices last year."

Tim Wolfenden, head of home services at consumer advice website uSwitch.com, said: "Price rises have been on the horizon for some time.

"Ever since British Gas and npower announced price increases on their market trackers and npower followed this up with a hike on its online plan, it's been clear prices are going to be heading north - with the smart money on a 15 per cent rise."

He said npower's decision to up standard rates will "give competitors the green light" to follow suit.